The present invention relates to a transducer and, in particular, a flexible, unitary electret film transducer for converting mechanical stress, such as mechanical vibrations, into electric signals, and to a method for its fabrication. The transducer is especially applicable for use in musical instruments, such as stringed musical instruments (guitars etc.), accelerometers and alike.
Saddle transducers i.e. pickups for acoustic guitars, designed to transform string vibrations into electric signals, are mounted under the saddle of the guitar. They have a transducer part of a length corresponding to that of the saddle and typically containing different layers of electromechanical transducer elements, dielectric material and electrically conductive electrode layers, and a connection cable part in which the signals are taken to a preamplifier inside the guitar via a small hole (diameter typically 3 mm) bored in the guitar""s resonance box under the saddle. Saddle transducers may typically have a one or more transducer element layers. Contact pickups are also commonly used for amplifying musical instruments sound. Typically they consist a piezo ceramic disk. Contact pickups pick up the sound from vibrating surface to which it is attached by means of glue or putty.
As electromechanical transducer elements, piezoelectric crystals or piezoelectric sheet (e.g. polyvinylidene fluoride PVDF) are prior art. In the commonest transducer structures, the connecting cable part is implemented using screened coaxial cable, which is connected to the electrode layers of the transducer part by soldering. Such a transducer is presented e.g. in U.S. Pat. No. 5,319,153. A drawback with this type of structures is the difficulty of fabrication of the transducer and relatively high manufacturing costs, because much of the work has to be done manually. Moreover, the connections to the preamplifier generally have to be made by soldering, because no connectors of sufficiently small size to go through the hole provided under the saddle are available for coaxial cables and because the connection between the transducer itself and the cable makes it impossible to mount the transducer from below. In addition, piezoelectric crystals and sheets are associated with a certain characteristic sound that is not quite in keeping with the guitar""s own acoustic sound. Further, the prior art saddle transducers structures comprise many material types, which affects to the sound produced by the saddle transducer.
The electret field, or the permanent electric charge, is achieved by injecting charges into dielectric material.
A dielectric porous electret film and manufacturing process for same, applicable for use as electromechanical material for a stringed musical instrument transducer, is described in U.S. Pat. No. 4,654,546, said dielectric film comprising permanently charged, biaxially oriented, foamed, usually homogenous film layer containing flat lens-like, shredded or cavitated gas bubbles which can also be called as voids or cells. The term xe2x80x9cdielectric cellular electret filmxe2x80x9d is used here to refer to generally porous type electromechanical films having a permanent electric charge injected into material.
WO-publication 96/06718 presents a procedure for pressure inflation of a pre-foamed plastic film, that makes it possible to manufacture strongly foamed film products, involving a high foaming degree and allowing the thickness of the product to be increased without increasing the amount of plastic material. The term xe2x80x9cdielectric swelled cellular electret filmxe2x80x9d is used herein to refer to a foamed film-like plastic product as described in that WO-publication and having a permanent electric charge injected into material.
The object of the present invention is to eliminate the drawbacks of prior art and achieve an improved transducer of a completely new type, in which a dielectric swelled cellular electret film is used to transform the string vibrations into electric signals instead of piezoelectric films or crystals. Flat lens-like gas bubbles in the electret film effectively limit the mobility of electret charges in the dielectric material, because the gases have an electric resistance five decades better than the best solid insulating materials have. At the same time, compared to hard structure of piezoelectric materials, they act as an elastic soft layer during the conversion of for example string vibrations into electric signals allowing pressure variations caused by vibrations to cause microscopic changes in its thickness. The change in thickness causes change in capacitance and produces an electrical output voltage in proportion to the sound source.
A further object of the invention is to produce a new type of transducer which, due to its elastic cellular structure, is capable of converting mechanical stress, such as string vibrations, into electric signals which, when converted into sound, compared to prior art piezoelectric saddle transducers or contact pickups, better correspond to the instrument""s own acoustic sound and allows playing at high volumes before feedback. Because of the elastic porous structure, the young""s modulus of the material is significantly lower and thus the impedance matching with wood is better than with piezoelectric materials. This results in natural sound similar to instruments own acoustic sound without any harshness or xe2x80x9cquackingxe2x80x9d as typically with piezoelectric materials.
Still another object of the invention is to produce a transducer which is of a construction thin enough to permit installation without changing any parts of the instrument, e.g. making the saddle lower, and which, when installed, does not affect the instrument""s own acoustic sound, and is as easy to install as possible without soldering.
Still another object of the invention is to produce a stringed musical instrument transducer capable of converting the vibration of each string separately into an electric signal.
A further object of the invention is to produce a transducer as simple as possible, having no separate transducer part and no separate conductor for connecting it to a signal processing device, but which has a unitary, flexible and laminated structure and in which the connections for connecting it to a preamplifier can be disposed sequentially or side by side and which in itself is able to produce a balanced signal (differential transducer) according to the attached claims.
A further object of the invention is to produce a new kind accelerometer type contact pickup.
This kind of transducers can be very economically fabricated for example by screen-printing the required electrodes with silver paste on sheets of dielectric film (e.g. polyester) and/or directly to electret film, placing several electrodes side by side on the same sheet. By laminating such sheets and dielectric cellular electret film, preferably swelled, on top of each other so that charged dielectric cellular electret film is only placed on a desired area at one end of the sheet while the other end is provided with a connector part with different electrode layers side by side, a laminate sheet is obtained from which the transducers can be cut out e.g. by punching. After that, it is only necessary to join a suitable connector to the electrodes at the connector end of the transducer by pressing mechanically.
With this method, it is possible to produce ultra thin and flexible transducers of desired length, design, shape and width, in which the electrodes in the transducer part are continuous extending from the transducer part to the preamplifier and which are unitary, flexible and thin laminate in construction. Fabrication is faster and more economic than with conventional methods.
The structure of the invention thus allows the application of an effective and economic production technique, not only for under saddle transducers, but also for contact transducers.
In one embodiment of the invention, no dielectric firm plastic layer, where the young""s modulus value typically is significantly higher than with cellular electret film, to carry the conductive electrodes, would be needed in the transducer structure adjacent to instrument saddle. Thus the transducer becomes thinner and the acoustic properties become excellent because the firm plastic layers are not absorbing and dampening the vibration energy. Further, because of saved thickness exclusive firm plastic films, the amount of transducer elements can be increased, without adding too much thickness, and thus the output voltage and therefore the signal-to-noise ratio are further improved. Further, due possible increase in thickness of elastic soft dielectric cellular layers the structure becomes softer which improves the string-to-string balance. Even further, in this embodiment the electrodes become more durable than screen-printed electrodes and the connectors in the preamplifier end can be easily connected to the transducer so that the there is no plastic layers in between and thus the electrical properties of connections become excellent and also more durable. Further, it is possible to simultaneously arrange the screening for the connection end and even soldering directly to the electrodes.
The structure of the invention thus allows the application of an effective and economic production technique with significantly improved sound and string-to-string balance properties.